Cherrium

Meet Cherrium, a particle who breaks the light-speed limit in water and leaves a blue glow behind.

Nothing can beat the speed of light in a vacuum. But light slows down in water, to c/n, where n ≈ 1.33. A fast charged particle can outrun that slower light. As it does, it disturbs the water around it and sends out a cone of light, much as a jet makes a sonic boom.

The cone's angle obeys cosθ = 1/(nβ), where β = v/c. For a particle moving at nearly c in water, cosθ ≈ 0.75, so θ ≈ 41°.

The glow only starts once β is bigger than 1/n ≈ 0.75. For an electron that means a kinetic energy of about 0.26 MeV. Faster particles make wider cones, and the light is strongest at short wavelengths, which is why it looks blue.

The glow lights up the water around nuclear reactor cores. Giant underground tanks use it to spot neutrinos.